The single-atom tip, with its special geometric shape and physical features, may be used as key component for advanced scientific equipments, such as scanning probe microscopy, field-emission electron microscopy, electron holography, electron energy spectroscopy, electron diffraction or interferometer, electron lithography, ion gun, low-energy electron microscopy etc. It may also be used in commercial electronic products such as flat panel display.
The term “single-atom tip” pertains to a metal tip, ending with one atom at its apex. Because there is only one atom at the apex, the single-atom tip is an ideal scanning probe. Take the scanning tunneling microscope (STM) as an example. In order to realize optimal spatial and spectral resolution, the probe of a scanning tunneling microscope must satisfy two major conditions: The tip must have an apex of one atom and the atomic structure of the tip must be known. If a single-atom tip with a known structure can have sufficient thermal and chemical stability, optimal spatial and energy resolution in the surface electronic structure may be obtained by using a scanning probe with such a metal tip.
Because its tip is terminated by one atom, the single-atom tip may serve as ideal field-emission electron source or ion source. In the application of the field-emission electron microscope, in order to improve the resolution and brightness of the electron microscope, an electron beam from a point source is necessary. An electron beam emitted from a metal tip ended with one atom has a narrow electron energy distribution. The electrons are highly coherent in phase also. The electron beam has a narrow extension angle and is thus brighter. Resolution of the microscope may thus be improved. Since the electrons are coherent, the beam can be used for electron holography to obtain the three dimensional image of a sample. The single-atom tip has a smaller onset voltage and may be used in consumer products such as field-emission flat panel display. In addition, such a metal tip may also be used in the applications of liquid metal ion source and field ionization ion source, so that optimal spatial and energy resolution of the related image may be obtained from the liquid metal ion source.
Although the single atom tip is very useful, it is difficult to prepare a single atom tip for commercial applications. In the conventional researches, several methods for preparing single atom tips were disclosed. Some used high electric field gradient to produce directional surface mobility to generate protrusion terminated with one atom. Some used high-energy electrons to heat metal tips, combined with field forming effect, to generate single atom tips. Some used field-surface-melting or ion bombardment, in combination with field evaporation and vacuum sputtering, to place a single tungsten atom on top of a small tungsten (111) facet. Although the above-mentioned methods could successfully produce a single atom tip, they need to be operated under ultra-high vacuum environment. The conventional preparation of the single atom tip is complicated and with poor yield rates. The single atom tip so prepared does not have a thermodynamically stable structure either, thus the operation lifetime is short and regeneration of single atom tips is very difficult or impossible. The metal tips so prepared don't have a well-defined structure of atoms at their apex and applications of such metal tips outside the ultra-high vacuum system are not possible. These drawbacks hinder the single atom tip from commercial applications. Such single atom tips have so far been used only in the laboratories.
Another approach to prepare single atom tips according to the conventional art includes the following steps: A tungsten tip is prepared and is cleaned by field evaporation under ultra-high vacuum. About one to two monolayers of palladium is deposited on the surface of the tungsten tip by thermal evaporation. After a 700□/3-5 minute annealing, due to the faceting of the Pd atoms covered W (111) surface, a pyramid structure is formed. This pyramid has a well-defined structure and is terminated by one atom. The most important advantage of this approach is that, when the tip is destroyed, the single atom tip can be regenerated after annealing the metal tip at 700□. This feature may satisfy the basic desirable properties of the single atom tip. A typical approach of such conventional method may be seen in Fu et al.: “Method of creating a Pd-covered single-atom sharp W pyramidal tip: mechanism and energetics of its formation”, Physical Review B, 64, 113401 (2001).
Although the noble metal induced faceting is a thermodynamic process, take the example of the pyramidal structure formation induced by faceting of Pd on W (111) surface, the following conditions shall be satisfied. The W (111) surface shall be clean, containing no oxides or any other impurity atom. The Pd layer shall be close to a physical monolayer. Faceting shall be induced on the W (111) surface after being heated in the range of 780-1100 K for 3-5 minutes. In the study by Fu et al., all preparation procedures were carried out under ultra-high vacuum environment to fulfill the above requirements. This is not convenient for most applications of single-atom tips. Especially, preparation of a clean W (111) surface requires a high electric field and the deposition of Pd layer requires an evaporator.
Therefore, it is desirable to provide a novel method to fulfill the above requirements. If preparation of a clean W (111) tip surface and deposition of noble metal films on the tip can be operated under the ambient environment, one would only need to heat the tip to obtain a single-atom tip in the vacuum system that uses a single-atom tip. Annealing is a simple process and can be found in most of the systems. Therefore, commercial applications for the single atom tip will be possible.